The file is completely customizable, and sets both the light values in game and the colors used in the toolset clickable icons to select said light colors.

The first column is the index, between 0-31 (for a total of 32 available slots)

RED GREEN and BLUE set the light color, and accept values between 0.00 and 2.00, and also negative values, used to subtract a certain color from the final result. It means that if you want to obtain a stronger BLUE color you can add negative values to RED and GREEN, but don't overdo it, as negative light values can quickly generate unwanted artifacts and trash your area lighting altogether.

LABEL is unused by the game or toolset, it's just for human eyes.

TOOLSETRED, TOOLSETGREEN and TOOLSETBLUE set the color of the icon in the toolset. Valid values are between 0.00 and 1.00, and you cannot use negative values.

The matching between light color and toolset color is arbitrary, and must be manually ensured by entering the appropriate values. You could associate a red light value with a green icon in the toolset, if for some weird reason that is the desired result.

Make sure that the first entry (ID 0) is set to black, so that the default tile light is no light.

Also, take into account that environment.2da (and area files) reference tile lights by ID, so if you change the light values in lightcolor.2da you'll be affecting already existing areas